In general, the present invention relates to the removal of contaminants from strongly alkaline solutions. More specifically, it relates to the removal of chloride and carbonate contaminants from solutions containing at least 6% sodium oxide content.
U.S. Pat. No. 4,041,135 discloses a method for producing crystalline base exchange aluminosilicate materials having high total ion exchange capacities. An essential step in that method is treating aqueous process liquids containing alkali metal aluminate to remove halide impurities, organic discoloring complexes and ferruginous sludge, and adjusting to the desired concentration to form clarified aqueous process liquids to serve as a source of alkali metal aluminate in the initial reaction.
These impurities need to be removed because the commercial grade raw materials of that invention are typically produced in processes which give rise to halide (usually chloride) and carbonate contamination of the raw materials. Unless these impurities are removed, they will accumulate in the recycle stream and reach levels where they cause undesired reactions to occur.
These impurities can be removed by membrane-type cells, fractional crystallization, or similar units. They can also be removed by draining off some of the recycle stream to a waste dump, but this wastes useful process liquids.